Nowadays, for a liquid crystal display panel in a normally black mode, the bright point cannot be effectively detected when the liquid crystal display panel displays the black image. As shown in FIG. 1, the signal characteristics of the existing black detecting screen are applying a higher voltage to the gate electrode (Gate) of the detected liquid crystal display panel to turn on the thin film transistor (TFT) switch of the detected liquid crystal display panel; applying a lower voltage (which is equal to the voltage on the common electrode (Common) of the detected liquid crystal display panel) to the data line (Data) of the detected liquid crystal display panel. At this time, the image of the liquid crystal display panel is black. Generally, during the manufacturing process of the liquid crystal display panel, contaminants in the pixel region or film-layer residues 2 as shown in FIG. 2 may occur due to tiny dusts, which results in a short-circuit between the pixel electrode 1 and the data line 4, thereby the voltage on the pixel electrode 1 is not controlled by the TFT switch, the voltage is equal to the voltage on the data line 4, and therefore the display is not synchronized with the pixel therearound, which causes displaying defects (usually shown as bright points under a gray-scale image).
However, since the voltage of the data line 4 is relatively low, there is no significant difference between the voltage of the pixel electrode 1 and that of other pixel electrodes therearound, which results in the gray-scaled displaying of the liquid crystal display panel without difference, and thus the bright point cannot be detected effectively.